This invention relates to etching of features in a semiconductor wafer, and in particular to a method of establishing precise mask alignments in the fabrication of such features.
Micromachining of silicon, which involves formation of small features by anisotropic etching of a silicon wafer or substrate, is gaining increasing importance in a number of device areas. Since feature size and shape rely upon the varying etch rates along different crystal planes, alignment of the photolithographic masks used for etching can be quite critical. For example, V-shaped grooves are presently formed in a (100) oriented silicon wafer to provide channels for alignment of optical fibers. (For a general discussion of etching silicon wafers having a (100) surface, see, e.g., U.S. Pat. No. 3,765,969, issued to Kragness et al, and assigned to the present assignee, which is incorporated by reference herein.) In order to provide grooves with the precise width needed for fiber alignment, the etch mask opening must be closely aligned with the (110) direction of the wafer. For some applications, the width error is desirably no greater than 0.25 .mu.m, which requires that the mask opening edge make an angle of no greater than approximately 5.times.10.sup.-3 degrees with the (110) direction. While the flat portion of silicon wafers is nominally in the (110) plane, this accuracy is no greater than .+-.0.5 degrees.
Consequently, it is an object of the invention to precisely establish the location of crystal planes in a semiconductor wafer and provide a means for aligning photolithographic masks with the appropriate crystal orientations to produce the desired features.
In many micromachining operations, such as the fabrication of silicon microphones and ink jet nozzles, it is necessary to etch the back surface of the silicon wafer to define the desired feature (see, e.g., U.S. patent application of Busch-Vishniac et al, Ser. No. 469,410, filed Feb. 24, 1983, and assigned to the present assignee). This type of fabrication usually requires an alignment of photolithographic masks on the front and back surfaces of the wafers so that the etched feature is properly aligned with elements formed by etching or other processing using photolithographic masks on the front surface. Such an alignment is typically produced by means of holes drilled through the substrate by lasers, While effective, this alignment technique is only as accurate as the mechanical positioning of the table on which the wafer is mounted during drilling. An alignment using photolithographic techniques would be far more accurate.
It is therefore a further object of the invention to provide front-to-back mask alignment in the etching of features in at least one surface of a semiconductor wafer.